Features/writermongo

Implement a JsPath for mongodb crud syntax

``` scala
(__ \ "foo" \ "bar").writemongo[Int].writes(1)
```

will generate the following JsObject
``` javascript
{
  "foo.bar":1
}
```

Which is pretty great if you wrap everything in another JsObject like:
```scala
Json.obj("$set" -> (__ \ "foo" \ "bar").writemongo[Int].writes(1))
```

``` javascript
{
  "$set": {
    "foo.bar":1
  }
}
```